Domain join uses YUM instead of DNF

The domain join plugin uses the yum command to add prerequisites for RHEL, RHEL-like, and Fedora distributions, but DNF replaced YUM with RHEL 8 in 2019 and and Fedora 22 in 2015. While many installs include the yum command as a symlink to dnf, it's not guaranteed to be there. Considering all supported versions of these operating systems use DNF, the scripts should be updated to use DNF.
